This invention relates to a focus indicating device which operates to indicate the contrast signal of the image of an object directly in the view-finder of a camera thereby to indicate the state of focusing.
A variety of focus detecting methods have been proposed. A so-called automatic focusing range finder type camera in which the lens is automatically moved to the focus position has been provided and is known in the art. However, an automatic focusing single-lens reflex camera has not yet been put in practical use. The main reason for this is that, since a single-lens reflex camera is considered to be used for professional photography, there is a strict requirement that the focus position is detected with high accuracy. Manual focus techniques are used and since the lens of the single-lens reflex camera is relatively heavy, the lens drive device is bulky. Also, since the lenses are interchangeable, a motor drive for each lens is not economical. In addition, in order to stop the lens at the focus position, it is necessary to first decide whether the lens is ahead or behind the focus position. The decision processing circuit is accordingly located in the camera body, for use with all lenses and is relatively intricate. These factors make the manufacture of the automatic focusing single-lens reflex camera difficult.
A focus detecting method has been proposed, which utilizes the fact that the optical intensity of each spatial frequency of an object image (the power spectrum) becomes maximum at the focus position. For instance, a method of electrically or mechanically scanning the image of an object by using a photoelectric transducer is known in the art. In this method, a time series scanning output signal equivalent to the brightness distribution of the image of an object is obtained by scanning the image, and then the signal is subjected to differentiation to extract a high frequency component which becomes contrast information. After the differentiation signal is converted into an absolute value waveform, the peak value in one scanning period of the absolute waveform is extracted so as to be utilized as a signal to indicate the state of focusing.
Recently, a method has been proposed in which a self-scanning type photoelectric element is used as the scanning means. A focus indicating device according to this method can be readily incorporated in a single-lens reflex camera, because it can be made compact in view of the employment of the self-scanning type photoelectric element. The above described peak value reaches a maximum when the proper focus is obtained. However, it is necessary to provide a detector for deciding that the peak value is at a maximum, in order to stop the lens at the focus position. For this purpose, heretofore two photoelectric elements are disposed before and behind the focal plane on the optical axis. Alternately, a memory circuit is used to subject to comparison two signals corresponding to two lens positions ahead and behind the focal plane.